bindings: python check in configury needs to be improved

[hppritcha]

There's a check in ompi_configure_options.m4 for python 3.6 or higher to support generation of the top level c and fortran f08 bindings from the code in ompi/mpi/bindings. That's fine, but the file being used as a sentinel to tell whether or not the bindings have already been generated is not correct. the names of the generated files has changed and ompi_send.c is not being generated, rather a differently named file.

[jsquyres]

I also think we probably need an explicit check in OMPI's configure for Python. At the moment, we have a Sphinx setup macro which requires python, that's only if you're building the docs (which obviously is optional).

Also right now, the only python required for a tarball build is for the Fortran MPI bindings (per #13231). So we could protect the python check behind a few things:

• if you're building the fortran MPI bindings, or
• if you're in a git clone that doesn't have various sentinel files, etc., or
• ...?